Question

What is true about cyanobacteria?
A. Oxygenic with nitrogenase
B. Oxygenic without nitrogenase
C. Non-oxygenic with nitrogenase
D. Non-oxygenic without nitrogenase

Answer 1

Hint: Cyanobacteria, commonly known as blue-green algae, was formerly classified as a plant by some biologists due to the presence of chlorophyll pigment similar to plants. This autotroph microorganism also capable of nitrogen fixation is later classified as eubacteria or true bacteria.

Complete answer:
Option A Oxygenic with nitrogenase
Bacteria show a wide variety of nutrition, and they are broadly classified into autotrophs and heterotrophs. Autotrophic bacteria are classified into chemoautotrophs and photoautotrophs, which are further divided into oxygenic and anoxygenic photoautotrophs. Cyanobacteria are oxygenic photoautotrophs, which means these bacteria reduce carbon dioxide into organic compounds in the presence of light and release oxygen as a byproduct. This is very much similar to plant photosynthesis. The photosynthesis in cyanobacteria is achieved by chlorophyll a pigment, and other light harvesting pigments called phycobiliproteins.
Certain filamentous cyanobacteria also act as nitrogen fixers. But nitrogen fixation that converts atmospheric nitrogen into ammonia requires an anaerobic condition because the nitrogenase enzyme that catalyzes the process is irreversibly inactivated by oxygen. Thus, nitrogen fixation in cyanobacteria is carried out in nitrogen-fixing cells called heterocysts, which have a thick cell wall, devoid of oxygen releasing photosystem II complexes, and presence of oxygen scavenging proteins.
Option A is correct
Option B Oxygenic without nitrogenase
Cyanobacteria or blue-green algae are oxygenic photoautotrophs that have the ability to do nitrogen fixation with the nitrogenase enzyme.
So, option B is incorrect.
Option C Non-oxygenic with nitrogenase
Cyanobacteria are photoautotrophs that release oxygen during photosynthesis and are known as oxygenic bacteria.
So, option C is incorrect.
Option D Non-oxygenic without nitrogenase
Cyanobacteria are oxygenic photoautotrophs and nitrogen fixers. Thus, they release oxygen during photosynthesis and also contain nitrogenase enzymes.
So, option D is incorrect.

So, the correct answer is “Option A”.

Note:
During photosynthesis (light reaction), the electron lost from the excited chlorophyll molecule in photosystem II is replaced via water splitting. This decomposition of water tends to release oxygen. On the other hand, cyanobacteria that fix nitrogen contain oxygen-sensitive nitrogenase enzymes. Thus, the nitrogenase enzyme is protected from the atmospheric oxygen and oxygen released by the photosynthetic cells via heterocyst cells.